Compiler Structure. Lexical. Scanning/ Screening. Analysis. Syntax. Parsing. Analysis. Semantic. Context Analysis. Analysis.

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Griselda Patterson
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1 Compiler Structure Source Program Text Phases of Compilation Compilation process is partitioned into a series of four distinct subproblems called phases, each with a separate well-defined translation task This structure is the result of more than 40 years of experience in compiler engineering Can be run in sequence, in which case they are called passes Or can be integrated in various ways, such as parallel execution as co-routines Parse Tree Annotated Parse Tree Lexical Analysis Token Stream Syntax Analysis or Postfix Token Stream Semantic Analysis or Intermediate Code Code Generation Target Machine Code or or or Scanning/ Screening Parsing Context Analysis

2 Phase 1: Lexical Analysis Lexical Analysis Lexical analysis, or scanning/screening, partitions source text into the words or tokens of the input language Source Program Text Lexical Analysis if x>5 then > keyword "if" identifier "x" operator ">" integer "5" keyword "then" Token Stream

3 Syntax Analysis Phase 2: Syntax Analysis Syntax analysis, or parsing, groups tokens together into the grammatical structures of the language, often represented as a parse tree or (equivalently) a postfix token stream Token Stream Syntax Analysis identifier "x" operator ">" > integer "5" op ">" id "x" int "5" Parse Tree or Postfix Token Stream identifier "x" integer "5" operator ">"

4 Phase 3: Semantic Analysis Semantic Analysis Semantic analysis, or context analysis, analyzes and verifies the meaning of the structures, and checks for semantic legality Parse Tree or Postfix Token Stream op "+" Semantic Analysis int "2" string ""hi"" Annotated Parse Tree or Intermediate Code > ERROR

5 Phase 4: Code Generation Code Generation Generates target machine code for analyzed structures May include optimization of structures, operations and code Annotated Parse Tree or Intermediate Code Code Generation Target Machine or Assembly Code

6 Alternate Phase 4: Interpretation Bytecode Interpretation Simulates the virtual machine of the intermediate code in software Executes the intermediate code directly Intermediate Code Bytecode Interpretation Execution

7 Merging Phases Compiler Architectures Some compilers merge or integrate one or more phases together (although they are still designed separately) to gain efficiency If all phases are merged, we have a one pass compiler One pass compilers can be very fast, but they can also be much more complex and difficult to maintain Refining Phases In order to reduce complexity and increase maintainability and reliability, very high quality production compilers (e.g. IBM s) often to the opposite - they actually split phases into even more independent sub-tasks For example, the IBM PL/I compiler actually has about 40 separate phases, each a separate pass

8 Tables Why is it called a Compiler? Language processing involves compiling tables of information about the program, to be used to analyze scopes, look up references and determine meaning - hence the term compiler Such tables may be shared between phases of compilation, using databases or disk files Or in other compilers each phase builds it own tables Either way, a big part of compiler engineering is the design of the table structures to support efficient analysis

9 The PT Pascal Compiler PT Pascal Originally designed and implemented by J. Alan Rosselet (U. of Toronto) about 1980, as a proof of concept for the S/SL compiler technology (then a new research result) Specifically designed for teaching and learning about compilers Designed to be easily portable (Pascal was at that time the standard portable language, as C is now) - it takes an expert about three days to port PT to a new machine (mostly spent understanding the new machine s assembly language) Designed to be real - generates good quality code for target machines, can compile and run itself, the S/SL Processor, and other quite large real programs Self-compiling - implemented entirely in PT Pascal itself, using S/SL

10 PT Compiler Structure Four Phase, Three Pass Scanner / Screener run as co-routine with Parser in Parser pass Parse tree represented as postfix stream of tokens Special PT abstract (virtual) machine code ( T-code ) PT Pascal Source Text Scanner/ Screener Parser Postfix Token Stream Semantic Analysis T-code Code Generation x86 Assembly Code or (Scanner/Screener and Parser run as co-routines) ( E.G. x + y > x y + ) (Intermediate virtual machine code designed for PT) T-code VM Simulator Interpetive execution Intel x86 Assembler x86 Machine Code (Not part of PT compiler)

11 Scanner PT Phase Overview: Scanner Breaks up input text into PT language tokens, ignores blanks, newlines etc. PT Pascal Source Text Scanner/ Screener Parser Postfix Token Stream Input to Scanner a :=b+ 11; Output from Scanner pidentifier "a" pcolonequals (The p prefix on token pidentifier "b" names indicates that pplus they are to be Parser pinteger "11" input stream tokens) psemicolon

12 PT Phase Overview: Screener Screener A filter between the Scanner and the Parser Recognizes keywords, evaluates literals, builds identifier table PT Pascal Source Text Scanner/ Screener Parser Input to Scanner const x=22; Output from Scanner pidentifier "const" (before screening) pidentifier "x" pequals pinteger "22" psemicolon Postfix Token Stream Output from Scanner/ pconst Screener to Parser pidentifier 273 (after screening) pequals pinteger 22 psemicolon

13 Parser PT Pascal Source Text Scanner/ Screener Parser Postfix Token Stream PT Phase Overview: Parser Context-free syntax checking (syntax error detection) Converts expressions to expression trees in postfix form, recognizes statement structure, disambiguates operators Input to Scanner/Screener a := b + 11; Input to Parser from pidentifier 632 Scanner/Screener pcolonequals pidentifier 243 pplus pinteger 11 psemicolon Output from Parser sassignmentstmt sidentifier 632 (The s prefix to token sidentifier 243 names indicates that sinteger 11 they are Semantic pass sadd input stream tokens) sexpnend

14 PT Phase Overview: Semantic Analyzer Semantic Phase Checks semantic constraints (e.g., type checking) Constructs symbol table, analyzes scopes, resolves references Generates T-code representation of program Postfix Token Stream Semantic Analysis T-code

15 PT Phase Overview: Semantic Analyzer Input to Semantic Analyzer sassignmentstatement from Parser sidentifier 632 sidentifier 243 sinteger 11 sadd sexpnend Output from tassignbegin Semantic Analyzer tliteraladdress 120 tliteraladdress 150 (PT virtual machine tfetchinteger bytecode - T-code) tliteralinteger 11 taddinteger tassigninteger

16 The PT Interpreter T-code Virtual Machine Simulator The T-code output by the Semantic Analyzer can be executed directly by a PT VM Simulator, to make a PT Interpreter T-code T-Code VM Simulator (Interpretive execution)

17 Code Generator PT Phase Overview: Code Generator Translates the T-code form of the program into machine code for the target computer Our target computer is the Intel x86, and the code generator outputs Linux x86 assembly code T-code Code Generation x86 Assembly Code

18 PT Phase Overview: Code Generator Input from tassignbegin Semantic Analyzer tliteraladdress 120 tliteraladdress 150 tfetchinteger tliteralinteger 11 tadd tassigninteger Output from Code Generator movl u+150, %eax addl $11, %eax (x86 assembly code) movl %eax, u+120

19 Summary Compiler Structure Compilers split into four phases: Lexical, Syntactic, Semantic, Code generation Phases may be implemented as separate passes, or integrated for performance PT Compiler implements four phases in three passes, using token streams between passes Uses PT virtual machine T-code as intermediate code References Text, chapter 2 Next Time The Syntax/Semantic Language (S/SL) - a domain-specific language (DSL) for specifying compiler phases

Today s Topics. Last Time Top-down parsers - predictive parsing, backtracking, recursive descent, LL parsers, relation to S/SL

Today s Topics. Last Time Top-down parsers - predictive parsing, backtracking, recursive descent, LL parsers, relation to S/SL Today s Topics Last Time Top-down parsers - predictive parsing, backtracking, recursive descent, LL parsers, relation to S/SL This Time Constructing parsers in SL Syntax error recovery and repair Parsing